Utilization of hydrophobic coatings on insulative skirts to attenuate galvanic corrosion
between mechanically-fastened aluminum alloy and carbon-fiber reinforced polymermatrix composites
Raghu Srinivasan*, L. H. Hihara
Hawaii Corrosion Laboratory, Department of Mechanical Engineering, University of Hawaii at
Manoa, Honolulu,
Hawaii 96822, USA
*Corresponding author. Tel.: +1 808 956 9412; fax: +1 808 956 2373.
E-mail address: raghu@hawaii.edu (Raghu Srinivasan)
Abstract
This paper describes the effect of hydrophobic coatings on insulative skirts to attenuate galvanic
corrosion between mechanically-fastened aluminum alloy and carbon-fiber reinforced polymermatrix composites (CFR PMC). The utilization of hydrophobic coatings on the insulative skirt
can help to attenuate galvanic corrosion by disrupting the formation of a continuous electrolyte
film. Initially, Siloxel™-coated insulating skirts of various lengths (0.64 cm, 1.27 cm, 2.54 cm,
10.16 cm, and 20.32 cm) were tested in a humidity chamber at 90% relative humidity (RH) and
30oC. The galvanic couples were sprayed with salt solutions of various concentrations (i.e, 10,
100, 1000, 10000, and 20000 ppm of chlorides). Results from humidity-chamber experiments
showed that the Siloxel™ coatings with a contact angle of 90° generally reduced galvanic
corrosion rates by three orders of magnitude for a given skirt length. When the skirt (coated with
Siloxel™), however, was at an optimal length of only 0.64 cm, the galvanic current decreased by
approximately six orders of magnitude due to capillary effects that wicked electrolyte away from
the edge of the skirt. Skirts with different types of coatings (i.e., Siloxel™, polyurethane, epoxy,
and latex) of various levels of hydrophobicity were also studied to determine the effect of the
coating contact angle on the galvanic current.
Keywords: Galvanic corrosion, Aluminum alloy, Carbon-fiber reinforced Polymer-Matrix
Composites, Siloxel™.
© 2015. This manuscript version is made available under the Elsevier user license
http://www.elsevier.com/open-access/userlicense/1.0/
1. Introduction
In a quest to reduce weight and improve performance, aluminum (Al) alloys have been used
in conjunction with lightweight materials such as carbon-fiber reinforced (CFR) polymer-matrix
composites (PMCs) in a multitude of applications in the defense, aerospace, automotive and
civil-infrastructure industries [1-5]. The direct contact of CFR PMCs with Al alloys can result in
accelerated galvanic corrosion of the Al alloys [6, 7], where the CFR PMC serve as the cathode
and the Al serves as the anode [8-11]. In chloride-containing solutions, oxygen reduction and/or
hydrogen evolution cathodic reactions can take place on the electrically conductive carbon fibers
that have corrosion potentials more positive than that of Al alloys [9, 10]. A typical method used
to mitigate galvanic corrosion when bolting Al to CFR PMCs is to insert an insulating layer (G10 fiberglass) between the members [6]. Although the direct contact between the Al and CFR
PMC is eliminated by the insulating member, the Al and CFR PMC can still be galvanically
coupled if the fastener is conductive (e.g., titanium bolt), and electrolyte bridges over the
insulating layer (Figure 1a) [9]. Galvanic corrosion can be more effectively mitigated by
increasing the ohmic loss (IR loss) between Al and CFR PMC by extending the insulation layer
(referred to as a skirt) beyond the perimeter of the CFR PMC (Figure 1b). The buildup of a
continuous ionically-conductive electrolyte film on the skirt, however, reduces the effectiveness
of the skirts (Figure 1b). The ohmic loss can be increased by breaking up the continuous
electrolyte film/salt bridge on the insulative skirt (Figure 1c) by using hydrophobic coatings.
Hydrophobic (contact wetting angle more than 90°) and super hydrophobic (contact wetting
angle more than 150°) [12-14] coatings have been synthetically synthesized on metal surfaces for
corrosion protection/inhibition due to their water repellent/self-cleaning property [15-19]. In this
present work, a novel approach of utilizing coatings (on insulating skirts) with varying degrees
of hydrophobicity was studied to attenuate galvanic corrosion between mechanically coupled
6061-T6 Al alloy and CFR PMC in accelerated corrosion tests.
2. Materials and Methods
2.1 Sample Preparation
Galvanic samples were prepared with a CFR PMC cathode area of 1.61 cm2 and a 6061-T6
Al anode area of 9.61 cm2. The cathode and anode areas were kept constant for all galvanic
specimens, although the insulating skirt length was varied. The insulating skirts were G10
fiberglass with a 0.55 mm thickness. For the uncoated skirt experiments, the G10 surface was
abraded with 180 grit silicon carbide grinding paper. For Siloxel™-coated insulating skirts,
lengths of 0.64 cm, 1.27 cm, 2.54 cm, 10.16 cm, and 20.32 cm were used. Siloxel™ is a highsilicon hybrid ceramic-polymer (or ceramer) coating synthesized using the sol-gel method [2022]. A fixed length of 1.27 cm was used for a set of specimens with skirts coated with Siloxel™,
epoxy, polyurethane or latex. Specimens were fabricated in triplicate and tested for each type of
specimen. The epoxy coating was commercially available and consisted of pichlorohydrin and
bisphenol A polymers (DOW Chemical, USA) with Ancamide 2325 curing agent (Air Products,
USA) [23, 24]. The polyurethane coating CONATHNE CE-1155 (CYTEC, USA) was a twocomponent, solvent-based system. The latex paint was flat white, water-based, and 100% acrylic
(Morwear ® Paint Company, USA).
2.2 Measurement of Galvanic Currents
The galvanic currents (IGalv) were monitored over a 3-day period inside a humidity chamber
at 90% relative humidity (RH) and 30oC. The IGalv between the CFR PMC and 6061-T6 Al was
recorded every 15 seconds by measuring the voltage across a 330 Ω resistor using a Madgetech
101A-160 mV voltage logger. The maximum voltage recorded was less than approximately
1 mV, and hence the introduced IR loss while measuring the galvanic current was minimal. The
couples were sprayed with different concentrations of NaCl salt solutions (i.e., 0, 10, 100, 1000,
10,000 and 20,000 ppm Cl-) for 15 seconds. Reagent grade NaCl salt and ultrapure 18.1 MΩ-cm
water were used to prepare salt solutions of different concentrations. For every length of
insulating skirt and salt spray conditions, the IGalv values were the average from three sets of CFR
PMC and 6061-T6 Al couples. The steady state values were the average current values from the
last 10 hours of the 72-hour humidity-chamber, galvanic-corrosion experiments. The average
steady-state IGalv values were normalized with respect to the 6061-T6 Al anode area and reported
as iGalv in A/cm2. The iGalv between CFR-PMC and 6061-T6 Al couples with uncoated insulative
skirt of zero length were also monitored as a control. In the zero-cm skirt specimens, the
CF PMC was separated from the 6061-T6 Al by the skirt thickness. A detailed description of
materials, sample preparation, and experimental procedures for humidity galvanic-corrosion
experiments can be found in an earlier work [11].
2.3 Determination of Superficial Hydrophobicity
Hydrophobicity of the insulating skirt with different coatings was determined by measuring
the contact angle for wetting using a KSV Instrument’s CAM 200 program. Contact angles were
measured using ultrapure 18.1 MΩ-cm water droplets in static mode at ambient temperature
(≈20°C). The wetting angles were calculated using the Young/Laplace curve fitting method [19].
The contact angles were measured from the left side and right side of the droplets and averaged
for 10 droplets on each substrate.
2.4 Model
The model chosen for this study was developed by Srinivasan et al [11]. Experimental
analyses of the galvanic corrosion mechanism of the CFR PMC and 6061-T6 Al were used to
develop a model linking the galvanic corrosion rate (iGalv) to salt loading (m’) and length of the
insulating skirt (ls). For a given salt loading (m’), the ohmic loss increases as the skirt length (ls)
increases. The ohmic loss is expressed as a function of m’ and ls [11] and it is given by
Log IR= Log
+ Log
, (1)
where iGalv is the galvanic corrosion rate, L is exposed anode length, WNaCl is the atomic weight
of NaCl, F is Faraday’s constant, Z is the magnitude of charge on the ionic species, and µj [cm2
sec-1 V-1] is the mobility of species j [11].
3. Results and Discussion
3.1 Determination of Galvanic Corrosion Current
The steady-state iGalv for the uncoated skirts gradually decreased as the skirt length
increased (Figure 2a), whereas the values for Siloxel™-coated skirts reached a minimum at
approximately 0.64 cm (Figure 2b). For the following conditions, the steady-state galvanic
current dropped to zero and could not be plotted on the log scale: a) uncoated skirts for chloridespray levels less than 100 ppm, b) 20.32 cm Siloxel™-coated skirts for all chloride levels, and c)
all SiloXel™-coated skirts for chloride levels less than 1000 ppm. The Siloxel™ coating
generally reduced iGalv by at least three orders of magnitude when compared to the uncoated
skirts. The Siloxel™ coating was hydrophobic and had a wetting contact angle of approximately
90°. This hydrophobic nature of Siloxel™ coatings disrupts the continuity of the electrolyte layer
on the surface of the insulating skirt and can reduce galvanic corrosion rates compared to skirts
without hydrophobic coatings. For short skirt lengths (e.g., 0.64 cm), the iGalv values reached a
minimum and were approximately six orders of magnitude lower. The hydrophobic nature of the
coated skirts aided the capillary forces that pulled the electrolyte to the CFR PMC surface or to
the border between the skirt and 6061-T6 substrate. The wicking effect was more pronounced
when the water globules resulting from the salt spray were larger than approximately the length
of the insulating skirt and this effect is diminished as the skirt length is increased to lengths
longer than the globule size.
3.2 Dependence of the Contact Angle
Skirts with different types of coatings (i.e., Siloxel™, polyurethane, epoxy, and latex) were
further studied to determine the effect of the contact angle on iGalv. A skirt length of 1.27 cm was
selected to study the effect of the contact angle on the formation of a continuous electrolyte film
without the full influence of the wicking effect. The 1.27 cm skirt length, which was twice the
length that led to the minima in igalv, was selected to reduce the capillary effects discussed above.
Contact angle measurements (Figure 3) showed highest values for the Siloxel™ coated G-10
fiber glass (92.5° ±5.06) followed by epoxy (77.3°±12.31), polyurethane (66.1° ±11.59), no
coating (55.8° ±2.37), and Latex (25.1° ±12.72). Siloxel™ was developed as a corrosion
protection coating for Al alloys and has hydrocarbon in its formulation to increase its
hydrophobicity [20]. The measured contact angle of 92.5° was slightly higher than the value of
80°±3 reported in the literature for pristine silicone sol-gel coatings [25]. The images of the water
droplets on the coated surfaces (Figure 3) show that the higher the degree of hydrophobicity
(e.g., Siloxel™) the more the electrolyte layer would tend to break-up; whereas, the lower the
contact angle (e.g., latex) the more likely it is that a continuous electrolyte layer will form.
During the galvanic corrosion experiments, the Siloxel™ coating lead to the formation of separate
globules on the skirt while the latex paint evenly distributed the salt spray over the skirt.
Accordingly, the steady-state igalv values (Figure 4) between 6061-T6 Al and CFR PMC followed
a general trend corresponding to that of the contact angles. The Siloxel™ coating with the
highest contact angle resulted in the lowest igalv, and the latex coating with the lowest contact
angle resulted in the highest igalv. For the highest chloride treatment (20,000 ppm Cl-), igalv for
the Siloxel™ coating dropped to 1.72 x 10-9 A/cm2, which was one to two orders of magnitude
lower than the values corresponding to the epoxy and polyurethane coatings, and three orders of
magnitude lower than that corresponding to the latex coating.
4
Conclusion
A novel approach of utilizing hydrophobic coatings on insulating skirts was demonstrated for
the attenuation of galvanic corrosion between mechanically coupled 6061-T6 Al and CFR PMC.
Hydrophobic coatings can render insulating skirts much more effective by preventing the
formation of a continuous salt–bridge layer. Galvanic corrosion was reduced by three to six
orders of magnitude compared to uncoated insulating skirts. The maximum attenuation of six
orders of magnitude occurred when water globules (resulting from the salt spray) were larger
than approximately the length of the insulating skirt (≈ 0.64 cm), and capillary forces aided by
the hydrophobicity of the coating wicked electrolyte away from the edge of the skirt. Contactangle measurements also showed that galvanic corrosion decreased as coating hydrophobicity
increased. These findings can be used to design strategies to eliminate or attenuate galvanic
corrosion in high-performance systems that are composed of a multitude of dissimilar materials.
Acknowledgements
The authors are very grateful for the support of the projects entitled “Corrosion and Corrosion
Control Studies of Aluminum Alloys that are Mechanically-Coupled or Adhesively-Bonded to
Polymer-Matrix Composites in Diverse Micro-Climates” funded by the USAFA ( FA7000-12-20014). The authors are particularly grateful to Daniel Dunmire, Richard Hays, Michael
McInerney, Gregory Shoales, Christopher Scurlock, Larry Lee, William Abbott, and David
Robertson with the Technical Corrosion Collaboration sponsored by the Office of the Under
Secretary of Defense. The authors also like to thank Dr. Atul Tiwari for developing and
providing the Siloxel™ coating. The U.S. Government is authorized to reproduce and distribute
reprints for Governmental purposes notwithstanding any copyright notation thereon. The views
and conclusions contained herein are those of the authors and should not be interpreted as
necessarily representing the official policies or endorsements, either expressed or implied, of the
US Air Force Academy or the U.S. Government.
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Figure Captions
Figure1: The schematic of galvanic corrosion between CFR PMC and 6061-T6 Al couple with
0 cm insulating skirt (a), insulating skirt >0 cm (b), and insulating skirt >0 cm with hydrophobic
coatings (c).
Figure 2: Steady state galvanic current between 6061-T6 Al and CFR PMC with different salt
sprays for various lengths of insulating skirts with no coatings (a) and Siloxel™ coatings (b) at
90% RH and 30oC.
Figure 3: Average contact angle measured on different coated G-10 fiber glass surface.
Representative water droplets are shown in the inserts for each surface.
Figure 4: Log of steady state galvanic current density between 6061-T6 Al and CFR PMC with
different coatings for 1.27 cm G-10 fiber glass skirt length.
Figure 1
Figure 2
Figure 3
Figure 4
Graphical Abstract